The pumps of this type comprise a centrifugal impeller made of a hub from which a plurality of blades having a curved shape extend. The centrifugal impeller is coupled to an electric synchronous motor contained inside a motor casing which is sealed in order to prevent water from entering inside, thus damaging the motor. The electric synchronous motor comprises a stator made of an electromagnet and a rotor formed by a permanent magnet which is integrally and axially coupled to the centrifugal pump. The centrifugal pump is housed inside an impeller casing which has a cylindrical shape defined bye a cylindrical wall and two circular side walls: a first circular side wall wherein a suction intake for the liquid to be pumped is made in the axial position and a second circular side wall defined by the motor casing. An outlet for the liquid is made in the cylindrical wall.
The impellers with curved blades are unidirectional, that is they have a predetermined rotating direction for a correct functioning, unlike impellers with straight and radial blades which are bidirectional. In fact, the latter have a symmetrical-axial impeller and, then, the rotating direction has no effect on the functioning of the pump.
As stated above, the centrifugal pumps are coupled to an electric synchronous motor which, as it is known, may start indifferently in one direction or in the opposite direction. Therefore, it is evident that, in case of impellers having curved blades, there are good chances that the pump begins to rotate in the wrong direction, thus preventing the pump from functioning or even starting.
Therefore, for the above-mentioned applications, impellers with straight blades are used. However these impellers have a low efficiency.
In the case of impellers with curved blades, different solutions have been adopted in order to always allow a correct starting of the pump, such as to make the free ends of the blades of the impeller with flexible materials which can be folded only in one direction, thus allowing a correct starting of the pump.
Although, these and other solutions are efficient, they significantly increase the cost of the product, above all, in consideration of the fact that the elements involved are small or even very small, any constructive complication negatively affects on the production time and then on the final cost.
Another aspect to be considered is due to the fact that such solutions may reduce the proper functioning of the pump, so causing losses of the liquid to be pumped and then to diminish the total efficiency of the same, that in pumps so small is already low.
It is evident that the low efficiency of a pump, with straight or curved blades, forces the dimension of the impeller to increase, and also the dimension of the electrical motor coupled to it; since it is necessary to have an oversized motor, the dimension of electro-pumps become remarkable.
Since the pumps are used in applications wherein the overall dimension has to be restricted, both due to the available space but, above all, in order to avoid a negative visual impact, the dimension of electro-pumps is an important characteristic, if not the main one.